Publications

Issue Archive

Optical Thin Films on Complex Substrate Geometries

Friday, 01 March 2013

Page 2 of 2

Materials

Unlike sputter or evaporative processes
which use solid targets as source materials,
CVD processes utilize a wide variety
of chemical compounds typically
referred to as precursors. There are 3
major classifications of precursors within
the CVD world: metal hydrides, metal
halides, and metal-organics. Within the
realm of metal-organic CVD (MOCVD),
there exist a large number of potential
precursor materials. In fact, there are
over 100 metal-organic compounds
available just for tin. Selection of an
appropriate precursor material is a key
aspect of LPCVD deposition. Important
considerations in the selection of a
source precursor material include the
reaction temperature, purity requirements,
reaction pathways, and the ability
to suitably vaporize the material and
deliver it to the substrate surface.

Proper handling of precursor source
materials and by-products is an important
aspect of CVD, as some materials
can be hazardous to people, animals,
and the environment. Abatement is
often achieved through the reaction of
by-products at very high temperatures,
followed by chemical scrubbing, absorption,
and/or condensation techniques
to separate by-products from the effluent
stream.

A wide range of materials have been
deposited using CVD techniques. These
include metal oxides, transparent conductive
oxides, nitrides, carbides, semiconductors,
pure metals, and synthetic
diamond. Accordingly, the number of
potential applications is immense.
Coatings produced by CVD can be used
in the aforementioned multi-layer interference
stacks as protective coatings for
diffusion, corrosion, and wear resistance,
and in a variety of photovoltaic, semiconductor,
and fiber-optic-based systems.

Transparent Conductive
Optical Coatings

A wide array of transparent conductive
oxides (TCOs) have been deposited
via CVD including fluorine-doped tin
oxide (SnO:F), aluminum-doped zinc
oxide (ZnO:Al), antimony tin oxide
(SnO:Sb), and indium tin oxide (ITO),
to name a few of the more common
materials. TCOs are characterized by
excellent transmission in the visible
range, while possessing significant electrical
conductivity. Due to the abundance
of free carrier electrons, these
materials can be highly reflective at
infrared and longer wavelengths when
sheet resistance is suitably low.
Applications include electrodes, antistatic
coatings, energy efficient low-E
glass, and RF-blocking coatings for security
applications. The use of such materials
on substrates of complex shape may
be an area for future growth, providing
new options for system design.

Applications

DSI’s IsoDyn LPCVD process has been
successfully employed to coat a wide
variety of different surfaces including
parabolic concentrators, dome optics,
ball lenses, optical fibers, tubes, and
other non-planar substrates. DSI has
considerable experience in the telecom/
datacom industry with high quality,
conformal AR coatings on ball lenses
(Figure 3). The AR coatings produced
by the LPCVD process provide 100%
coverage on each ball lens, eliminating
any orientation considerations. This, in
turn, provides for a reduction in the
manufacturing costs associated with the
assembly of optical fiber couplers/collimators
— typically operating in the
wavelength range between 1.30μm to
1.57μm.

Today, ball lenses are finding use in
new fields beyond fiber coupling. One
area of increasing interest is in the field
of concentrated photovoltaic (CPV)
power generation. Placed at the center
of an array of mirrors, a large-diameter
ball lens can be used to focus solar
energy onto a high-efficiency solar cell.
This system takes advantage of the optical
properties of ball lenses to collimate
diffuse light into a tightly focused
beam. The application of an antireflective
coating can be used to
improve system efficiency by up to 6.5%
through minimization of reflectance
losses. The IsoDyn LPCVD process is
capable of coating ball lenses ranging
in size from 200μm to 200mm with single,
dual band, and broadband AR coatings,
as well as more complex optical filter
designs.

The LPCVD process has also been
employed by DSI in a variety of reflector
applications covering a wide range of
component geometries. The coatings
deposited on reflectors range from
broadband visible cold mirror designs,
shortwave pass, longwave pass, and
dichroic filters. The capability exists to
handle part sizes up to 8" × 12".

The new capability to produce coatings
covering the spectral band range
from 300nm to 5μm, encompasses a wide
range of optical thin film designs. In the
visible range, broadband AR, shortwave
pass, longwave pass, cold mirror (Figure
4), and dichroic filter designs have been
successfully employ ed. Single, dual, and
broadband AR coatings are also available
for telecom wavelengths of 1310nm/
1550nm on substrates ranging in index
from BK7 to sapphire and cubic zirconia.
Further more, broadband solar (400nm
to 1700nm) AR coatings as well as hot
mirror may be applied to a variety of substrates.
In some cases, it may be possible
to achieve performance over multiple spectral regions, such as an AR coating
for both VIS and midwave infrared
(MWIR) spectral bands.

Thin film optical coatings produced
with the proprietary LPCVD process can
be used in extremely harsh operating
environments. The coatings produced
by this process are thermally stable and
chemically inert in most operating environments,
with demonstrated service
temperatures up to 850°C. Mechanical
durability and adhesion are excellent, a
result of the strong covalent bonds with
the substrate and at layer interfaces.
The IsoDyn process is useful for coating
almost all optical glasses, crystalline
materials, ceramics, and metals.

Summary

The advanced IsoDyn coating process
utilizes the fundamental advantages of
LPCVD deposition in order to provide
conformal, low defect thin film coatings
on non-planar and asymmetrical optical
components. The benefits of LPCVD
processing have long been exploited in
the semiconductor industry, and now
provide new opportunities for novel
optical system designs. New interference
filter coatings for single wavelength,
dual band and broadband AR,
cold mirror, hot mirror, long/shortwave
pass, dichroic, conductive are available.
The coatings deposited using the IsoDyn
process are extremely robust and capable
of performing in the most demanding
operating environments.

This article was written by David McLean,
Process Engineer Deposition Sciences, Inc.
(DSI) (Santa Rosa, CA). For more information,
contact Mr. McLean at solutions@
depsi.com, or visit http://info.hotims.com/45601-200.

Question of the Week

This week's Question: A recent study created by the Arizona-based Paragon Space Development Corporation says its life support system could help humans survive on Mars. The proposed Environmental Control and Life Support System, the company says,...